1. Field of the Invention
The present invention relates to an electrode assembly and a secondary battery including the same, and more particularly, to an electrode assembly in which a separator formed of a ceramic layer is interposed between a positive electrode plate and a negative electrode plate and a secondary battery including the electrode assembly.
2. Description of the Related Art
Generally, a secondary battery can be used repeatedly if it is charged. The secondary battery has been used as a main power source of portable devices for communication, information processing and audio/video. Recently, the secondary battery has been rapidly developed because it has ultra-light weight, high energy density, a high output voltage, a low self-discharging rate, environment-friendliness and a long life as a power source.
The secondary battery is divided into a nickel-hydrogen (Ni-MH) battery and a lithium ion (Li-ion) battery according to electrode active material. Particularly, the lithium ion battery can be also divided into a lithium ion battery using a liquid electrolyte and a lithium ion polymer battery using a solid polymer electrolyte or lithium ion battery using a gel type electrolyte according to electrolyte type. Further, the lithium ion battery can be divided into various types such as a can type and a pouch type according to a shape of a container receiving an electrode assembly.
The lithium ion battery can implement an ultra-light battery because its energy density per weight is very higher than a disposable battery. An average voltage per a cell of the lithium ion battery is three times more than other secondary batteries such as a NiCad (Nickel-Cadmium) battery or a nickel-hydrogen battery. The self-discharging rate of the lithium ion battery is about ⅓ of that of the NiCad battery or the nickel-hydrogen battery. The lithium ion battery is environmentally-friendly because it does not use heavy metals such as cadmium (Cd) or mercury (Hg), and has an advantage in that it can be charged/discharged more than 1000 times under a normal condition. Accordingly, the lithium ion battery has been rapidly developed with the growth of an information and communication technology.
In a conventional secondary battery, a bare cell is formed by receiving an electrode assembly including a positive electrode plate, a negative electrode plate and a separator in a can made of aluminum or aluminum alloy, finishing an opening of an upper end of the can with a cap assembly, injecting electrolyte into the inside of the can and sealing the can. Because the can is made of aluminum or aluminum alloy, it has advantages in that it can be light-weighted by lightweight property of aluminum and does not corrode even when it is used for a long time under a high voltage.
The sealed unit bare cell is received in a separate hard pack with being connected to safety devices such as a PTC (positive temperature coefficient) device, a thermal fuse, a Protective Circuit Module (PCM) and other accessories. Or, its external shape may be formed by a mold made of hot melt resin.
The separator of the electrode assembly, which is an olefin type film separator, is installed between the positive electrode plate and the negative electrode plate in order to prevent an electrical short between two electrodes. However, when the separator existing between two electrodes does not have sufficient permeability and wettability, there is a problem that the separator restricts movement of lithium ions between two electrodes so that an electrical property of the battery is degraded.
Further, the olefin type separator functions as a safety device for preventing the battery from being overheated. However, in the case where the battery temperature is suddenly increased due to some reason, for example, external heat transfer, etc., the separator may be damaged by the battery temperature increased continuously for a predetermined time even though micro-pores of the separator are closed.
In addition, if capacity of the battery becomes higher by a high density active material layer and thus density of an electrode plate is increased, there is a problem that injecting speed of an electrolyte becomes low or the electrolyte is not injected by required amount because the electrolyte does not sink into the electrode plate.
When the battery is continuously charged/discharged, a by-product is produced by redox reaction between positive electrode and negative electrode active materials and the electrolyte and thus the electrolyte is continuously exhausted. Accordingly, if absolute amount of the electrolyte as a media for ion movement between the positive electrode and the negative electrode is not fulfilled, capacity of a cycle is decreased.
Additionally, when large current flows in the secondary battery in short time according to high capacity of the battery, there is a problem that possibility of the electrical short due to the separator damage is increased because the separator is continuously melted by previously generated heat, rather than the battery temperature being decreased by current shutdown even if micro-pores of the separator are closed.
According to a request for stably preventing the electrical short between the electrodes even under high temperature, the separator includes a ceramic layer including a porous membrane formed by combining ceramic filler particles with a heat-resistant binder.
The ceramic layer of the ceramic separator includes a plurality of pores. That is, a high rate charge/discharge property can be obtained by using ceramic powder of higher porosity than the conventional olefin type film separator. In addition, the ceramic layer rapidly absorbs the electrolyte. Accordingly, the injecting speed of the electrolyte is increased, thereby allowing productivity of the secondary battery to be improved.
However, size and distribution of the pore of the ceramic layer should be optimized. That is, lithium dendrite is easily educed between the pores if the size of the pore of the ceramic layer is too large. Thus, sub-reaction proceeds between the lithium and the electrolyte, and the battery is ignited or exploded due to heat and gas caused by the sub-reaction, thereby allowing safety of the battery to become worse.
On the other hand, if the size of the pore of the ceramic layer is too small, there is a problem that a lifetime or a high rate charge/discharge capacity or low temperature charge/discharge capacity are reduced because lithium ions cannot smoothly move.